Encapsulated radioactive light sources



Sept. 24, 1 968 c, H. CLAPHAM, JR 3,

ENCAPSULATED RADIOACTIVE LIGHT SOURCES Filed May 12, 1965 2 Sheets-Sheet 1 FIG. I

INVENTOR. CHARLES H. CLAPHAMJR ATTORNEYS l 1968 c. H. CLAPHAM, JR

ENCAPSULATED RADIOACTIVE LIGHT SOURCES 2 Sheets-Sheet 2 Filed May 12, 1965 FIG. 6

INVENTOR. CHARLES H. CLAPHAM JR ATTORNIEYS United States Patent 3,402,492 ENCAPSULATED RADIOACTIVE LIGHT SOURCES Charles H. Clapham, Jr., Berwick, Pa., assignor to United States Radium Corporation, Morristown, N..I., a corporation of Delaware Filed May 12, 1965, Ser. No. 455,153 6 Claims. (Cl. 40--130) ABSTRACT OF THE DISCLOSURE A combination of a device, such as a sign or dial, having a transparent body adapted to be illuminated and an encapsulated radioactive light source having a rigid housing forming a seat for carrying a transparent potting material in which there is imbedded a hollow glass element containing a phosphor and a radioactive beta-ray emitting gas.

This invention relates to radioactive light sources and, more particularly, to a separate encapsulated radioactive light source which can serve as a replaceable unit for various devices which are adapted to be illuminated.

In recent years, there has been a significant increase in the use of radioactive light sources in which a beta-ray emitting gas, such as tritium, activates a phosphor within a sealed glass tube or bulb. Since these are self-contained devices requiring no external source of power, they have found particular application as illuminating means for signs which must remain lighted under emergency condi-v tions, for example, in exit signs, aircraft and space capsule control panels, life raft instruction indicators, escape doors, and so on.

However, the glass element containing the phosphor and radioactive gas, which constitutes the light source itself, is a relatively fragile unit and because of the evident safety hazard presented it must be incorporated in the overall assembly in a fashion which eliminates any danger of leakage of the radioactive gas even under the most severe conditions of stress. It has therefore been the practice to integrate the glass tube into the assembly with a full complement of shock absorbing means, secondary gas seals, etc. which are as much a part of the entire assembly as they are of the glass element itself. Not only are these precautions a significant cost factor from the standpoint of material, but they require that the assembly of the complete illuminated device be carried out under the stringent safety standards applicable to loading and sealing of the glass element. For example, all personnel involved in the manufacture of the overall device must wear radioactivity-detecting badges and they must be given periodic physical examinations.

In addition, the accepted design practice of incorporating the gas-filled tube and its protective components into the overall assembly in an integral manner precludes stocking of extra tubes as replacement light sources. This is true not only because it is almost always impossible to remove a defective light source from a complete unit without destroying the unit itself, but also because the integrated design concept normally calls for a very specific and perhaps unique light source tube for every given illuminated device.

It is the primary object of this invention to isolate the light source tube from the remainder of the associated assembly both structurally and during manufacture so that it is, for all practical purposes, as much a separate sub-assembly as an electric light bulb. The purpose is to provide a specially encapsulated radioactive light source which, as a unitary sub-assembly, can be handled and assembled with other parts under ordinary factory safety 3,402,492 Patented Sept. 24, 1968 conditions, stocked as a replacement light source, and incorporated in or removed from the overall device with case, all consistent with the standards of safety required for the manufacture and operation of radioactive articles.

Broadly stated, the invention contemplates, in combination with a device adapted to be illuminated, at least one encapsulated radioactive light source. The light source comprises a protective housing defining an at least partially enclosed rigid seat from which light can be transmitted. Transparent elastomeric potting material is disposed within this seat. Embedded Within the potting material is a hollow sealed glass element containing a phosphor and a radioactive beta-ray emitting gas.

The housing may be a metal channel or a hollow plastic or glass tube, and it may be translucent or opaque. Where potting materials of a relatively high hardness and stiffness are used, they may themselves serve as the housing by being cast around the hollow sealed glass element. It is the function of the housing to encase the fragile glass element as compactly and protectively as possible within the potting material and yet permit the desired transmission of light. For the latter purpose, the housing may be specially shaped to control light distribution and those surfaces impinged upon by the light may be suitably treated to be reflective.

Such an encapsulated radioactive light source would itself be assembled, of course, under the accepted safety standards now applicable to all stages of production of an overall unit containing a beta-ray emitting gas in some one component. In other words, personnel responsible for fabricating the encapsulated light source and its parts would be required to wear detection badges and undergo the accepted routine of other specific health precautions. However, and in this sense the invention is often advantageous, the completed encapsulated light source could thereafter be handled in most instances with more relaxed safety measures because it is fully sealed, reinforced and protected against gas leakage even when very roughly handled. Consequently, it may be adapted to assembly line procedures with the many other parts with which it is combined to form a finished illuminated device, and the greater part of production operations may be carried out under practically normal shop conditions. It becomes, as it were, no more extraordinary as a component part than an incandescent bulb.

To the same extent, the encapsulated light source of the invention can serve as an interchangeable part adapted to be stocked as a replacement item. It can generally be inventoried and handled under common working conditions, and it may therefore be designed in standard sizes rather than to custom specifications. The overall illuminated devices in which the encapsulated light sources are to be used may also be designed to include them as easily insertable and removable parts, and several examples of such devices are described hereinafter. If a light source in one device proves defective, as sometimes happens, another standard size may be ordered to replace it, perhaps Without removing the device from its place of operation and certainly, without destructi'vely dismantling it.

All of these features have previously been unknown in radioactive light sources, and they are made possible by the separate encapsulation of the gas-filled element in its own compact shock-resistant potting material and protective housing.

Preferred embodiments of the invention are described hereinbelow with reference to the accompanying drawings, wherein FIG. 1 is an elevation partly in section of one form of the encapsulated light source of the invention;

FIG. .2 is an elevation partly in section and broken away of another form of the encapsulated light source of the invention;

FIG. 3 is an enlarged section taken along the line 33 of FIG. 1;

FIG. 4 is a section taken along the line 44 of FIG. 2;

FIG. 5 is a section which may be an alternate to that of FIG. 3;

FIG. 6 is an elevation partly in section of a sign equipped with the separate encapsulated light source of the invention;

FIG. 7 is a section taken along the line 77 of FIG. 6;

FIG. 8 is a perspective partly broken away of another sign equipped with the separate encapsulated light sources of the invention;

FIG. 9 is an exploded perspective of still another sign equipped with the separate encapsulated light source of the invention; and

FIG. 10 is a section of yet another sign equipped with the separate encapsulated light source of the invention.

In FIGS. 1 and 3, an encapsulated radioactive light source according to the invention is shown wherein the light source itself is a hollow elongated sealed tubular element 10 of glass. Disposed in a substantially uniform layer over the entire interior surface of the tubular element 10 is a particulate phosphor such as zinc sulfide, zinc cadmium sulphide, cadmium sulfide, cadmium tungstate or the like. The tubular element 10 is also filled with a beta-ray emitting radioactive gas such as tritium or krypton-85, usually under pressure. This ultimate source of light in the combination of the invention is self-activating and has a prolonged operating life, but it is relatively fragile and would constitute a hazard if the radioactive gas within it were to leak.

To provide a safe unitary sub-assembly which can be handled without stringent precautions, the tubular element 10 is housed entirely in a transparent plastic tube 11, preferably of polymethylmethacrylate, having a greater inside diameter than the diameter of the tubular element 10. The bore of the plastic tube 11 defines a cavity or seat within which the light source element 10 is protectively located and from which light can be transmitted in a plu rality of directions. To insure that the element 10 is isolated from shock and other stresses, the housing provided by the plastic tube 11 is filled with a transparent elastomeric potting material 12, such as a low-temperature curing silicone gel which can be poured into the bore of the tube 11 as a clear liquid containing a catalyst. To locate the element 10 concentrically within the tube 11 as the potting material 12 sets, a few small drops of a rapid curing elastomeric material (such as natural or synthetic rubber or plastic foam, or a translucent or opaque silastic) are applied to each end of the element 10 to form discreet cushions 13 holding the element 10 spaced from the bore of the tube 11. The tube 11 is longer than the light source element 10 and its ends are covered by caps 14 of the same material as the tube 11 which may be formed in situ or inserted as separate closures. Depending upon the desired function of this encapsulated light source, the tube 11 may be of circular cross section as in FIG. 3 or otherwise, as shown for example in FIG. 5 where its outer crosssectional configuration is square (primed reference numerals being used in FIG. 5 for the various components corresponding to those of FIG. 3).

Referring to FIGS. 2 and 4, another form of the encapsulated light source of the invention is shown. In this embodiment, a tubular glass light source element 16 is positioned by cushions 17 and embedded within shock resistant potting material 18 as in the embodiment of FIGS. 1, 3 and 5. Here, however, the protective housing consists of a metal channel 19, preferably of sheet aluminum which forms an open elongated seat from which light can be transmitted in one general direction (to the left as shown in FIG. 4. A layer of reflective white paint 20 may be disposed around the inside of the channel 19 to intensify the light output from the tubular element 16.

Again, the housing provided by the channel 19 is longer than the tubular element 16 and its ends are closed by a deposit 20 of elastomeric material such as that used for the cushions 17. As in the previous embodiment, the configuration of the tubular element and its housing may vary depending upon the circumstances of use, and in fact the light source element could be in the form of a substantially spherical bulb rather than a tube.

It is typical of these forms of the encapsulated light source of the invention that they constitute a compact and relatively safe sub-assembly having many of the attributes of an incandescent light bulb as an interchangeable production component. This is so because the tube 11 or channel 19 serving as the housing envelops the fragile light source 10 or 16 in a protective rigid casing isolating it from all stresses. Also, the potting material 12 or 18 within the housing protects the fragile glass tubular element from vibration and shock, and in addition serves as a secondary barrier guarding against escape of the radioactive gas in the unlikely event the glass element were to crack. The potting material may readily be formed around the tubular glass elements in a solid mass free of voids. It is also evident that each of these forms of the encapsulated sub-assembly can be manufactured quite cheaply since they pose few problems from the standpoint of assembly.

Furthermore, the encapsulated light sources of the invention typified by FIGS. 1 to 5 can be made in standard sizes and stocked as replacement items. Since they are compact and uncomplicated in shape, the various illuminated devices in which they are ultimately intended to function can be readily designed to receive one of a limited selection of standard sizes of the encapsulated light sources. Again, their self-contained safety features permit them to be stocked, shipped and installed as replacement parts.

Turning now to FIGS. 6 and 7, one form of an illuminated device is shown in Which these encapsulated light sources may be installed. This device is an edgelighted sign comprising a flat elongated transparent plastic body 25, preferably or polymethylmethacrylate. Along one side of the body 25 is an encapsulated light source as shown in FIGS. 2 and 4 comprising a metal channel 26, an inner layer of reflective white paint 27, potting ma terial 28 and the tubular glass light source element 29.

The open end of the channel is disposed in a direction facing toward the interior of the body 25 with a completely closed interface between the potting material 28 and the plastic surface of the body 25, so that the light is transmitted readily into the body of the sign. A metal casing 30 surrounds all of the body 25 and this encapsulated light source but for a window area 31 in which legend material 32 may be disposed. Suitable fasteners 33 are provided to hold the device together in a releasable fashion to permit the encapsulated light source to be removed if desired. Since the light from the encapsulated light source is directed into the body 25 of the sign parallel to the surface of the window area 31, this is what can be described as an edge-lighted sign. To assist in the distribution and concentration of light, a coating 32 of reflective white paint may be disposed over the entire inner surface of the casing 30.

Another illuminated device where the encapsulated light source of the invention proves useful is the Exit sign shown in FIG. 8 wherein two encapsulated light sources 35 and 36 of the type shown in FIG. 5 are inserted in. suitably configured holes extending transversely through a fiat elongated transparent plastic body 37 of the sign. A metal mounting channel or casing 38 is disposed partly around the body 37 leaving a frontal window area 39 from which light is transmitted, and again fasteners 40 are employed to hold the assembly releasably together. Legend letters 41 are disposed in this window area 39 in a manner such that the encapsulated light sources 35 and 36 are oilset between them to eliminate bands of corn centrated light apparent to the viewer.

In FIG. 9 another form of illuminated device in which the invention is applicable is shown and this comprises a plastic body covered with opaque paint except for a window area 46 on which legend material 47 is disposed. An encapsulated light source 48 of the type shown in FIGS. 2 and 4 may be fitted into a groove 49 in the backside of the body 45 and held in place by side and back plates 50 and 51 secured by fasteners 52 and 53. The channel of the encapsulated light source 48 has its open side directed toward the interior of the plastic body 45 and toward the window area 46 so that this is not an edge-lighted sign. it may have a less uniform distribution of light in the window area apparent to the viewer but it is more compact and therefore serves more appropriately under certain circumstances.

A final use of the encapsulated light source of the in vention is illustrated in FIG. 10 wherein an encapsulated light source 55 similar to that of FIGS. 2 and 4 is disposed against the edge of a flat elongated transparent plastic body divided by a central highly reflective light barrier 56 into two body elements 57 and 58. This composite body and the light encapsulated light source 55 is surrounded by a metal casing 59' with two opposed window areas 60 and 61 each of which may have its legend material. This then is a two-way edge-lighted sign wherein the encapsulated light source 55- provides the illumination for both window areas. As in the previous devices, it may be held releasably together by suitable fasteners which are not shown in the drawing.

Each of these illuminated devices wherein the encapsulated light source of the invention is combined may often be assembled under ordinary production conditions without regard to radioactive-detecting badges or other special health precautions for personnel, since the encapsulated light source sub-assembly in most instances can be treated as one of several components no more extraordinary than the others. If for some reason the light source of a given finished device proves defective, it may be replaced by dismantling the device in a simple fashion and ordering another such encapsulated light source of the same standard size.

I claim:

-1. In combination with a device having a transparent body adapted to be illuminated, at least one replaceable encapsulated radioactive light source and a rigid frame means providing for removably securing the light source adjacent said device, said source comprising (a) a protective rigid housing having an at least partially enclosed rigid seat thereby defining at least one opening from which light can be transmitted and directed toward the device;

(b) transparent elastomeric potting material disposed 'within said seat; and (c) a hollow elongated sealed glass element embedded within and completely surrounded by said potting material longitudinally within said seat and containing (i) a phosphor, and (ii) a radioactive beta-ray emitting gas.

2. A combination according to claim 1 wherein said housing is a metal channel and the elongated seat defined thereby is an open cavity from which light can be transmitted in one general direction.

3. A combination according to claim. 1 wherein said housing is a transparent plastic tube and the seat defined thereby is an interior cavity from which light can be transmitted in a plurality of directions.

4. A combination according to claim 1 wherein the device is a sign.

5. A combination according to claim 4 wherein the sign is edge-lighted and the open cavity is directed toward a portion of said casing.

6. A combination according to claim 4 wherein the sign is directly lighted and the open cavity is directed toward said window area.

References Cited UNITED STATES PATENTS 2,953,684 9/1960 MacHutchin et a1. 313-108 X 2,994,148 8/1961 Endelson 40-130 X 3,026,436 3/1962 Hughes 250-106 X 3,031,519 4/1962 Silverman 250-106 3,038,271 6/1962 MacHutchin et al. 250-106 X 3,197,902 8/1965 Buzan 40-130 3,260,846 7/1966 Fever 250-106 X LAWRENCE CHARLES, Primary Examiner. 

